SINGAPORE MEDICAL JOURNAL

DISACCHARIDASE DEFICIENCY AND OF CARBOHYDRATES

C K Lee SYNOPSIS

A large proportion of man's caloric intake is carbohydrate and starch and account for over three-quarters of the total consumed. But the rapid change of the food industry from an art to a high specialised industry in recent years have made available a variety of rare food , amongst which are various disacharides. Since the digestion or enzymic breakdown of carbohydrates is a normal initial requirement that precedes their absorption, and of carbohydrates varies according to their molecular structure, these rare sugars can cause diseases of carbohydrate intolerance and malabsorption. Intolerance and malabsorption can be due to polysaccharide intolerance because of dy- sfunction (caused by the absence of pancreatic ) or malfunctioning of absorptive process (caused by damaged or atrophied absorptive mucosa as a result of another primary disease or a variety of other casautive agents or factors, thus resulting in the Department of Chemistry inability of carbohydrate absorption by the alimentary system). A National University of Singapore third type of intolerance is due to primary deficiency or impaired Kent Ridge activity of digestive of the . The Singapore 0511 physiological significance and the metabolic consequences of such a , -isomaltase, mal- C K Lee, Ph.D., F.I.F.S.T., C.Chem. F.R.S.C. deficiency or impaired activity of Senior Lecturer tase and are discussed.

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INTRODUCTION abundance in the free state. It is a constituent of the important plant and animal reserve sugars, starch and Recent years have seen a considerable advance in all . It arises through the hydrolytic action of saliva facets of enzymology, not least in our understanding of and pancreatic J -amylase (d.-1, 4-glucan 4-glucanohy- the biochemistry, physiological role, nutritional signifi- drolase) on starch. In addition, maltotriose, isomaltose [6- cance, and industrial potential of the that 0-(d-D-glucopyranose) D-glucopyranose], and some hydrolyse , the disaccharidases. Under- branched tri-, tetra-, penta- and -saccharides are also standing of the chemical nature of these enzymes and its produced. A fourth dietary is or relation to catalytic activity has progressed to the point mushroom (d-D-glucopyranosyL. d-D-glucopyra- where the enzymes can increasingly be utilised to advan- noside). This naturally occurring non -reducing sugar is a tage in food and biotechnology processes, analysis and storage sugar of fungi, algae and pteridophytes. In the diagnosis and treatment of gastrointestinal disorder. mushrooms and yeasts, it accounts for as much as 15% of Over the years, with the growth and increased sophisti- dry weight and it can thus be of considerable importance cation of the food industry, significant changes have in single cell protein foods. taken place in the types of carbohydrates that are Food disaccharides, and consequently the disacchari- available in our food supplies. Consequently, various rare dases, the enzymes, that hydrolyse these sugars, have a food sugars are now found in manufactured foods. Since significant role in food and nutrition (1). The physio- the metabolism of carbohydrates varies according to their logical importance of these enzymes is self evident. molecular structure, disorders of carbohydrate intole- Disaccharides cannot be utilised as energy sources rance and malabsorption could arise from the presence of directly, but must first be hydrolysed to their component these sugars. prior to absorption. The metabolic consequences of a deficiency or imparied activity of FOOD DISACCHARIDES disaccharidases are serious and lead inevitably to clinical symptoms such as diarrhoea, vomiting, abdominal dis- Carbohydrates supply a large proportion of man's caloric tension and a general failure to thrive (1,2). However, the needs. In the diets of the poor, especially in the tropics problem of 'fermentative diarrhoea' received little interest they may form as much as 80% of the total calories while in until the later half of the 1950-s when Crane and his the diets of the rich, the proportion is smaller, but still coworkers (3) described a completely new model for the substantial. Much of this carbohydrate is in the di - events occurring during active transport of certain mono- sacharide form, the common ones being sucrose, , saccharides, thereby focusing interest on sugar digestion and , the breakdown product of starch digestion. and absorption in general. These sugars are too large to be absorbed and remains in the small intestine where they are ultimately hydrolysed DISACCHARIDASES to their component monosaccharides by the disacchari- dases located in the brush border of the mucosal epithe- General characteristics lial cells. Sucrose (a-D-glucopyranosy i. R-D-fructofuranoside) The glycosidic bonds are broken by a hydrolytic is widely distributed in the plant Kingdom and is parti- mechanism, involving a transfer of components of the cularly important in food processing. Lactose or milk - substrate to water. The action of carbohydrases generally sugar [4-0-(ß-D-glucopyranosyl) D -] is a natural is highly specific for a particular glycosidic bond linking constituent of milk, occurring either as the free sugar or in particular monosaccharides. Thus, sucrose can be cleav- lactose -containing oligosaccharides. Maltose [4-0-(4-D- ed either by anDC-glucosidase at position (a) or ß-fructo- glucopyranosyt) D -glucose], is not found naturally in furanosidase at (b) (Fig. 1). The configuration, d or ß,

Fig. 1 Mechanism of action of carbohydrases

HO glucose-[C(1)180H] +

(a)

OH + H218O

OH (b)

glucose + fructose-[C(2)18OH] Sucrose

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about the anomeric carbon atom is an important factor in pancreatic d -amylase All the oligosaccharides are then determining whether or not the saccharide will be cleaved hydrolysed to the monomeric sugars glucose, by a certain . For example, both maltose and and fructose by a group of disaccharidases associated cellobiose (4-0-(ß-D-glucopyranose) D-glucopyranose] with the brush border membrane (4). The classification of are disaccharides composed of two molecules of D - brush border enzyme activities (5) is show in Table 1. The glucose. However, cellobiose possesses a ß-glycosidic resulting monosaccharides are actively transported by linkage. Consequently, the two sugars are not cleaved by carrier systems (6,7) [although it was generally believed the same enzyme. The enzymatic hydrolysis of glycosides that fructose absorption is not by an active transport is essentially reversible, although in most cases the system (3)] into the cells and to the portal circulation. hydrolytic reaction is favoured. However, under suitable The disaccharidases of the intestinal mucosa hy- conditions, instead of water, the other drolyse d-glucosidic and ß-glucosidic linkages. The units present may serve as acceptors and lead to the actual activities of these enzymes are shown in Tables 2 formation of new disaccharides or even oligosaccharides. and 3 (8,9). The enzymes are particulate or insoluble The presence of these in significant amounts in foods will enzymes and they are not secreted into the lumen of the undoubtedly lead to metabolic problems. small intestine. Apart from trehalase (d, -trehalose 1-D- glucohyrolase, E.C.3.2.1.28), which hydrolyse only treha- activities lose, the other enzymes have rather broad substrate specificities. These enzymes exist in isoenzyme forms for Digestion of carbohydrates is initiated in the mouth and which the biological significance has not yet been es- stomach by the action of salivary d -amylase. In the small tablished. intestine, digestion is continued under the influence of

Table 1. Disaccharidases in the brush border of the human intestinal mucosa.

Enzyme or enzyme complex Substrate Products

Lactase- Lactose Glucose Galactose Hetero-ß- Galactose galactoside Cel lobiose Glucose Glycosylceramide Monosaccharide N-acylspingosine

Sucrase-isomaltase Sucrose Glucose Fructose Maltose Glucose Palatinose Glucose Fructose d --Dextrins Glucose (e.g. isomaltose)

Trehalase Trehalose Glucose

Two heat stable mallases Maltose Glucose Isomaltose Glucose

Glucoamylase (oligosaccharidase) Malto-oligo- Glucose saccharides (2-9 glucose units)

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Table 2. Disaccharidase activities in jejuna! mucosal cells (28).

Mucosa! protein Disaccharidase activity (per g protein) Subject content (mg/g wet weight) Isomaltase Sucrase Lactase

Normal subjects

1 94 500 123 144 39

2 94 502 110 109 89

3 78 1120 268 325 258

4 330 217 56 49 43

5 150 223 65 70 48

6 147 310 122 105 74

7 91 635 216 229 146

8 105 575 164 167 73

9 112 524 122 143 65 10 72 810 172 195 174

11 111 730 225 138 103

12 130 - 340 87 109 40

Normal subjects with low jejuna! lactase activity

101 86 990 283 255 7 102 103 410 92 115 17 103 112 458 129 139 17

104 91 670 181 188 20 105 130 618 158 234 12

Patients with milk intolerance

201 176 290 93 107 6 202 103 662 204 220 8 204 132 284 88 74 6

Table 3. Actual activities of disacchardases in normal mucosa (9).

Activity Enzyme (umol/min/g protein)

Maltase 308.8 + 95.2

Sucrase 101.5 + 30.3

Isomaltase 101.6 + 28.4

Lactase 55.8 + 21.9

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ENZYME DEFICIENCY AND MALASSIMILATION OF and subsequent diarrhoea. Bacterial action in the colon CARBOHYDRATES leads to the formation of a variety of products, such as carbon dioxide, hydrogen, lactic acid, acetic acid, Until recently, very little concrete information has been ethanol, glycerol and fatty acids. The most frequent available about impaired carbohydrate digestion and symptoms are frothy and explosive watery intermittent absorption as a cause of gastrointestinal disease. Most of diarrhoea, cramps, abdominal distension, audible bor- the evidence on the enzyme defects in disaccharidase borygmi, feeling of bloating, flatulence and possibly deficiency syndromes had been obtained indirectly with nausea and severe abdominal pain. blood sugar curves and faecal analysis after oral admini- stration of the sugars. More recently, methods for oral biopsy of the intestinal mucosa had been developed (10). When intestinal enzyme activity is absent, primary Approximately 70% of the world's adult population is deficiency or enzymopathy is found for lactase (ß-D- lactose intolerant (11). It appears that those who have galactoside galactohydrolase, E.C.3.2.1.23), sucrase, genetically acquired low lactase levels as adults are able isomaltase, palatinase or trehalase (E.C.32.1.28) (Table to drink milk as infants, but gradually become increasing- 4) This malfunction may be fatal if not diagnosed in time. ly lactose intolerant after infancy. We are, however, still A disaccharide intolerance is classified as primary when it unable to explain why a few ethnic groups have developed is observed with no history or sign of underlying intestinal the ability to retain high intestinal lactase activity disease (Table 4). These defects are inborn errors of throughout life but it is definite that lactase activity is not metabolism and manifest themselves as isolated defects regulated by adaptation to lactose in the diet of the single immediately at birth (as alactasia) or after the introduc- individual. Studies (12) have shown that lactase activity is tion of the offending disaccharide into the food (e.g. not changed by either lactose or any other sugars; sucrose). The intestinal mucosa is morphologically sucrase activity and those of others-, on the normal. These deficiencies persist through life other hand, are somewhat increased by certain sugars. Secondary deficiency of disaccharidases is caused by Lactose intolerance is the most widespread and best damage to the intestinal mucosa by any of a variety of understood type of carbohydrate intolerance. Primary causative agents or factors, such as viral, bacterial, drugs, lactase deficiency (hypolactasia) in adults is not a con- malnutrition, etc., leading to secondary intolerance of the genital defect. It is inherited in an autosomal recessive disaccharide (Table 4). The intolerance is often trans- way (13). Lactase activity gradually decreases during cient. Histologically the changes in intestinal mucosa growth of the individual to 5-10% of the starting activity. reflect abnormalities due to the underlying disorder. Normally, ingested lactose is hydrolysed by the enzyme Hypolactasia is the most frequent disorder. lactase, found within the brush border region of the Since only the liberated monosaccharides can be microvilli of the jejunal mucosa, to glucose and galactose, actively transported by the intestinal cell membrane, a which are then absorbed. If enzyme activity is low, deficiency in or an absence of any disaccharidase acti- ingested lactose is not hydrolysed. The intact disaccha- vities can result in elevated levels of one or more ride is too large to be absorbed and remains in the disaccharides in the colon. These disaccharides (and intestinal lumen. Here it acts osmotically to draw water tisaccharides, tetrasaccharides, etc.) would produce into the intestine (to dilute the hypertonic load), causing osmotic diarrohoea, causing an increase in the amount of abdominal distension, cramps and increased peristalsis. water in the lumen of the colon. The accumulated water In addition, the unabsorbed lactose is fermented by causes distension which provokes increased peristalsis certain anaerobic microflora inhabiting the small and

Table 4. The classification of carbohydrate malassimilation

Reduced disaccharidase Impaired monosaccharide Type activity transport (enzyme deficiency) (defect in membrane transport)

Primary Lactase deficiency Glucose -galactose malabsorption - congenital absence (congenital) (alactasia) - adult hypolactasia Sucrase-isomaltase deficiency

Trehalase deficiency

Secondary Lactase deficiency Transcient glucose -galactose malabsorption (acquired) Lactase deficiency with deficiency of other disaccharidases

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to lactic acid, which serves as a cathartic. isomaltase (25) involves (i) protonation of the glycosidic Carbon dioxide and hydrogen are also produced by this oxygen, (H) splitting of the bond between glycosy 1 C-1 fermentation and contributes to the frothy diarrhoea. and glycosidic oxygen, (Hi) formation of a carbonium ion A low level of milk consumption is often implicated as which is temporarily stabilised by a carboxylate group the cause of low lactase levels. However, milk drinking present in the enzyme's and finally by an OH - does not seem to affect lactase levels although some from water, with the return of the -configuration; both deficient' individuals can gradually increase their milk sucrase and isomaltase activities lead to retention of intake (12,14). On the other hand, prolonged abstinence configuration. from lactose in those individuals destined to have high Sucrase-isomaltase deficiency is an autosomal reces- lactase levels does not appear to inhibit lactase activity. sive trait (26), commonly encountered when the sufferer Alactasia, in contrast to hypolactasia, is probably a is in early infancy, especially immediately after being congenital disease (15) and evidence (16) suggests that weaned, but rarely in adult life. The symptoms of sucrase- this is also an autosomal-recessive trait. It is a condition isomaltase deficiency appear as soon as foods containing characterised by the complete absence of lactase in the sucrose are introduced into the diet. Deficiency normally intestinal mucosa. The consequence of this defect is that manifests itself in young infants as severe diarrhoea lactose is not hydrolysed and all the symptoms of lactose resulting in malabsorption syndrome and failure to thrive. intolerance are present from the first day of life, and In older infants or young children deficiency is indicated persist throughout life. Alactasia is, however, very rare by abdominal distension, borborygmi and diarrhoea and only about 30 cases have been reported in the produced by osmotic effect of unabsorbed oligosac- literature. charides and fermentation products (lactic acid, hy- In the absence of lactase, there is also always a lack of drogen and fatty acids such as butyric acid) within the glycosylceramidase (phlonzin ) (Table 1), bowel. which constitute the enzyme complex with lactase (17), Clinically, is more important than and this absence probably leads to steatorrhoea which isomaltose intolerance This is simply because, firstly, the accompanies the lactose intolerance. amount of isomaltose ingested is always very small since Cases of severe infantile gastrogenic lactose intole- there is only very small amount of isomaltose in the diet, rance have also been reported (18). This is a very rare arising mainly from starch and dextrin. Secondly, there disorder and is probably hereditary. It is not caused by are two other intestinal maltases (Table 1) so that any disorder in the disacchandase activities or by im- intestinal isomaltase activity is never reduced as much as pairment of monosaccharide transport but by abnormal sucrase activity. Thus intolerance of dextrins and starch permeability of lactose through the gastric mucosa. often disappears in many children in the course of the first Disaccharidas activity in the intestine is normal and, year or during the first few years of life and clinical therefore, unlike alactasia, intolerance is not accom- symptoms of starch intolerance are lacking in some panied by diarrhoea (18). The suffering infant vomits patients despite a deficiency of isomaltase activity.

excesively and becomes dehydrated (19). Lactosuria and The physiological, ' biochemical, nutritional and sucrosuria occur followed ultimately by liver damage. The medical aspects of the sucrase-isomaltase enzyme com- affected child often develops cataracts (18). All symptoms plex was recently reviewed by Cheetham (27). will disappear on a milk -free diet and in some cases milk tolerance may subsequently occur. Maltase defficiency Sucrase-isomaltase deficiency Congenital maltase deficiency will probably not occur, since in addition to the sucrase and isomaltase (both of Sucrase-isomaltase deficiency is a rare condition and is in which have maltase activity) there are two more maltasés fact a congenital defect in which the enzyme complex (el -1,4-glucan glucanohydrolase, E.C.3.2.1.20) in the consisting of two distinct subunits (20), each acting mucosa, and these have high enough activities to digest independently on its specific substrate, is deficient (Table dietary maltose or starch (Table 3). 1). The sucrase-isomaltase complex is composed of a maltase-isomaltase enzyme (E.C. 3.2.1.10) which can hydrolyse maltose, isomaltose, isomaltotriose, isomaltu- Trehalase deficiency lose, oligo-d-(1,6)-glucosides and oligo-,d-(1,4)- glucosides, and a maltase -sucrase (sucrase glucohy- Trehalase (a,oc -trehalose 1-D-glucohydrase, E.C.3.2.1. drolase, E.C. 3.2.1.48) which can hydrolyse sucrose, 28) is a specific enzyme, capable of hydrolysing maltose, maltotriose andel -(1,4) -glucosides. only trehalose (Table 1). Trehalase deficiency would, The sucrase-isomaltase complex is located chiefly in therefore, be expected and would manifests itself on the and proximal , being completely absent ingestion of young mushroom (containing up to 1.4% in the stomach and colon. The activities are greatest in the trehalose) (28,29). The deficiency is however not widely distal portion of the villi. The sucrase and isomaltase studied and to date only 7 cases have been described in enzymes appear to be subject to the same or very closely the literature. It is probable that this condition is more related biological control mechanisms. This is clearly common but is no doubt generally considered as intole- indicated by the constancy of the ratio of sucrase to rant to mushroom. isomaltase activities in random biopsy samples of human Trehalose is present in significant quantities in fungi intestinal tissue (21) (Table 2), their simultaneous ap- and various other microorganisms. Since some micro- pearance during development (22,23) and their absence organisms are potential protein protein sources for hu- from (4) or lack of activity in the brush border region of the man in the future it is, therefore, not impossible that with intestine of sucrose and isomaltose maldigesters (24). changing food habits, trehalase deficiency may become The mechanism of the hydrolytic action of sucrase and more important than it is at present.

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Disaccharidase deficiency and the Eskimos most cases. Three of the 19 subjects were, however, practically without sucrase or isomaltase activity while For a very long time the diet of the Eskimos is essentially two others had extremely low trehalase activity and three free of any carbohydrates. They, therefore, represents a others had moderately low trehalase activity. It thus particularly interesting group for study. A recent investi- appears that the 'inborn errors' with absence of one or gation (30) of the disaccharidase activities of a group of another of the intestinal disaccharidases, which occur as Greenland Eskimos showed, as expected, that nearly all very rare disease elsewhere, are rather common among the subjects were lactase deficient (Table 5). The two the Greenland Eskimos. There is little cause for concern subjects who had normal lactase activity were closely for these enzyme defects among the Eskimos since their related to a Dane who had migrated to Greeland and diet contains virtually no carbohydrates. However, it is married an Eskimo. Maltase, isomaltase, sucrase and puzzling why these 'mutants' should have become so trehalase activities, on the other hand, were normal in frequent among the Eskimos.

Table 5. Disaccharidase activities in jejuna) biopsies from adult Greenland Eskimos (28).

Disaccharidase activity (units per g protein) Subject Maltase Isomaltase Sucrase Trehalase Lactase

Lactose malabsorption

1 56 16 16 15 1.4 2 226 95 62 10 2.3

3 145 76 61 13 1.7

4 185 67 59 21 1.6 5 334 105 82 8.7 3.0 6 204 86 58 15 2.9 7 43 2.2 0.05 14 1.9 9 319 130 100 42 3.6 10 92 1.5 0.6 20 4.0

11 132 41 29 17 1.5 12 240 82 67 77 2.3 13 154 55 46 2.7 8.2 14 159 69 54 6.0 2.0 16 56 1.4 0.02 28 4.3

17 180 59 64 11 2.4

18 192 67 56 9.1 2.1 19 307 82 59 6.0 3.8

No lactose malabsorption

8 290 105 75 1.5 21 15 236 69 60 14 21

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REFERENCE:

Brush border membrane lactase. 1 Lee CK, Lindley MG, eds. Developments in Food Carbohy- 17. Cousineau J, Green JR: drates, 111, Disaccharidases. London: Applied Science Publi- Purification and Kinetic analysis of 3 forms of the enzymes shers, 1982. from the proximal and distal small intestine of suckling rats. 2. Dahlqvist A. Enzyme deficiency and malabsorption of car- Biochem Soc Trans 1979; 7: 961-962. Isolation and characteri- bohydrates. In: Supple HL, McNutt, KW, eds. Sugars in sation of the proximal and distal forms of lactase phiorizin Nutrition. New York: Academic Press, 1974: 187-214. hydrolyase from the intestine of the suckling rats. Biochim 3. Crane RK: Internal absorption of sugars. Physiol Rev 1960, 40. Biophys Acta 1980; 615. 147-57. 789-825. Wilson TH. Intestinal Absorption. Piladalphia: 18. Hirashima Y, Shinozuka S, leis T, Matsuda, I, Ono, Y, Murata, Saunders, 1962. T: Lactose intolerance associated with cataracts. Eur J A, J, Mrskos A, 4. Crane RK, Malathi P, Preiser H: A digestive -absorptive sur- Paediat 1979; 130: 41-45. Hoskova Sabacky face as illustrated by the intestinal cell brush border. Trans Pospisil R: Severe lactose intolerance with lactosuria and Amer Microsoc Soc 1975; 94: 529-44. vomiting. Arch Dis Child 1980; 55: 304-16 5. Gray GM. The physiology of intestinal absorption of sugar. In: 19. Berg NO, Dahlqvist A, Lindberg T. A boy with severe infantile Jeanes A. Hodge J, eds. Physiological Effects of Food gastrogen lactose intolerance and acquired lactase defi- Carbohydrates. Washington: ACS Symposium Series, Ameri- ciency. Acta Paediat Scand 1979; 68: 751-8. can Chemical Society, 1975: 181-90. 20. Asp N -G, Dahlqvist A: Separation of human intestine sucrase 6. Lutkic A, Votava A. Enzymic deficiency and malabsorption of from isomaltase FEES Letters 1973; 35: 303-5. food disaccharides. In: Lee CK, Lindley, MG, eds. Develop- 21. Auricchio S, Dahlqvist A, Murset G, Prades A: Isomaltose ments in Food Carbohydrates. 111. Disaccharidases. London: intolerance casuing decreased ability to utilise dietary starch. Applied Science Publishers, 1982: 183-211. J Paediat 1963; 62: 165-7. 7. Gracey M, Burke U, Oshin A: Intestinal transport of fructose. 22 Rubino A, Zimbalatti F, Auricdhio S: Intestinal disaccharidase Lancet 1970; 2: 1627-8. activities in adult and suckling rats. Biochem. Biophys Acta 8. Haemmerli UP, Kistler H, Ammann R, et al: Acquired milk 1964; 92: 305-11. intolerance in the adult caused by lactose malabsorption due 23. Dahlqvist A, Lindberg T: Development of the intestinal di- in human to a selective deficiency of intestinal lactase activity. Amer J saccharidase and alkaline phosphatase activities the Med 1965; 38: 7-30. foetus. Clin Sci 1966; 30: 517-28. 9. Lucking T, Wenner J: Activities of intestinal disaccharidases 24. Dubs R, Steinmann B Gitzelmann R: Demonstration of inac- and alkaline phosphates in jujunal biopsy in children. Eur J tive enzyme antigen in sucrase-isomaltase deficiency. Hely Paediat 1976, 121: 263-77. Paediat Acta 1973; 28: 187-98. 10. Dahlqvist A: Assay of intestinal disaccharidases. Anal Bio- 25. Walsh C. Enzyme reaction mechanism. San Francisco: W.H. chem 1964; 7: 18-25. Methods of assay of intestinal dis- Freeman 1979. accharidases. Anal Biochem 1968; 22: 99-107. 26. Kerry KR, Townley RR: Genetic aspects of intestinal sucrase- 11. Bayless TM, Paige DM, Ferry GD: Lactose intolerance and isomaltase deficiency. Aust Paediat J 1965; 1: 223-35. milk drinking habits. Gastroenterology 1969; 60: 605-8. 27. Cheetham PJ. The human sucrase-isomaltase complex: 12. Rosenwig NS, Herman RH: Diet and disaccharidases. Amer J Physiological, biochemical, nutritional and medical aspects. Clin Nutri 1969; 22. 99-102. In: Lee CK, Lindley, MG, eds. Developments in Food Car- 13. Dahlqvist A: The basic aspects of the chemical background bohydrates. 111. Disaccharidases. London: Applied Science of lactase deficiency. Postgrad Med J 1977; 53(suppl. 2-4); Publishers, 1982: 107-139. 57-62. 28. Bergoz R Trehalose malabsorption causing intolerance to 14. Keusch GT, Troncale FT. Miller FG, Promadhat V, Anderson mushroom. Gastroenterology 1971; 60: 909-12. PR'. Acquired lactose malabsorption in Thai children. Paediat 29. Madzarovova-Nohejlova J: Trehalose deficiency in a family. 1969; 43: 540-5. Gastroenterology 1973; 65: 13-33. 15. Holzel A, Schwaz V, Sutcliffe KW: Defective lactose absorp- 30. Asp N -G, Cook GC Dahlqvist A: Activities of brush border tion causing malnutrition in infancy. Lancet 1959; 1: 1126-8. lactase, acid Pi-galactosidase and hetero-ß-galactosidase in 16. Newcomer AD, Gordon H, Thomas PJ, McGill DB: Family the intestine of the intestine of lactose intolerant Zambian studies of lactose deficiency in the American Indian: African. Gastroenterolgy 1973; 64. 405-18. Gastroenterology 1977, 73: 985-8.

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